Liquid development by reducing the viscosity of the developer on a roller applicator prior to development

ABSTRACT

AN ELECTROSTATOGRAPHIC IMAGING SYSTEM OF THE LIQUID DEVELOPMENT TYPE WHEREIN A LIQUID DEVELOPER OF ELEVATED VISCOSITY IS INITIALLY SUPPLIED TO THE SURFACE OF AN ELECTROCONDUCTIVE DEVELOPER APPLICATOR DEVICE FOLLOWED BY THE APPLICATION OF A LIQUID CAPABLE OF LOWERING THE VISCOSITY OF THE DEVELOPER. THE APPLICATOR DEVICE IS THEN MAINTAINED IN MOVING CONTACT WITH AN IMAGING MEMBER BEARING AN ELECTROSTATIC CHARGE PATTERN TO PROVIDE A REPRODUCTION OF IMPROVED SOLID AREA COVERAGE.

Dec. 14, 1971 MASAMICHI SATO ETAL 3,Z7,57

LIQUID DEVELOPMENT BY REDUCING THE VISQOSITY OF THE DEVELOPER ON A ROLLER APPLICATION PRIOR TO DEVELOPMENT Filed Aug. 21, 1970 INVENTOR. MASAMICHI SATO SEIJI MATSUMOTO ATTORNEY United States Patent 3,627,557 LIQUID DEVELOPMENT BY REDUCING THE VIS- COSITY OF THE DEVELOPER ON A ROLLER APPLICATOR PRIOR TO DEVELOPMENT Masamichi Sato and Seiji Matsumoto, Asaka, Japan, assignors to Xerox Corporation, Stamford, Conn. Filed Aug. 21, 1970, Ser. No. 65,915 Claims priority, application Japan, Aug. 27, 1969, 44/67,749 Int. Cl. G03g13/10, 15/10 US. Cl. 117-37 LE Claims ABSTRACT OF THE DISCLOSURE An electrostatographic imaging system of the liquid development type wherein a liquid developer of elevated viscosity is initially supplied to the surface of an electroconductive developer applicator device followed by the application of a liquid capable of lowering the viscosity of the developer. The applicator device is then maintained in moving contact with an imaging member bearing an electrostatic charge pattern to provide a reproduction of improved solid area coverage.

BACKGROUND OF THE INVENTION This invention relates to imaging systems and more particularly to high-speed liquid development systems for developing electrostatic latent images.

The formation and development of images on the surface of photoconductor material by electrostatic means is well known. The basic xerographic process as taught by C. F. Carlson in US. Patent 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic marking material referred to in the art as toner. The toner will normally be attracted to those areas of the layer which retain a charge thereby forming a toner image corresponding to the electrostatic latent image. The powder image may then be transferred to a support surface such as paper and permanently affixed to the support by any suitable means such as heat fixing or solvent fixing. Alternatively, the powder image may be fixed to the photoconductive layer if elimination of the powder transfer step is desired. In addition, instead of latent image formation by uniform charging and followed by imagewise exposure, the latent image may be formed by directly charging the layer in image configuration. Other methods are known for applying electroscopic particles to the imaging surface. Included within this group are the cascade development technique disclosed by E. N. Wise in US. Patent 2,618,552; the powder cloud development technique disclosed by C. F. Carlson in US. Patent 2,221,776; and the magnetic brush process disclosed for example, in US. Patent 2,874,063.

Development of an electrostatic latent image may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with a charged image pattern the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration. Electroice phoretic development of an electrostatic latent image may for example, be obtained by pouring the developer over the image bearing surface, by immersing the imaging surface in a pool of the developer or by presenting the liquid developer on a smooth surface roller and moving the roller against the imaging surface. The liquid development technique has been shown to provide developed images of excellent quality and to provide particular advantages over other development methods in offering ease in handling. Liquid development is also capable of providing high development speed and the development speed of commercial embodiments has recently reached a level of as high as about 10 centimeters per second. However, with the currently available liquid development systems, this development speed is practical only for line copy since the development of continuous tone or halftone images generally requires a much slower speed.

In general, the increase in development speed is accompanied by various difficulties, such as low density of the developed image due to shortened development time. This reduction in image density, which is particularly marked in continuous-tone reproduction, can only be improved by prolonging the development period. A second drawback observed at higher development speeds is the formation of streaks in the developed image which resemble the tail of a comet. This streaking phenomenon, which has not been completely explained, does not appear, however, when the tangential speed of the liquid developer with respect to the image plane is small. Consequently, in order to prevent this streaking, it is necessary to minimize the relative tangential speed of the liquid developer with respect to the image plane. In addition, in the presence of a development electrode, this streaking phenomenon becomes more frequent with shorter distance between the image plane and the development electrode.

In the field of electrophotography, the liquid development process provides the best image quality in the reproduction of continuous tone image or multicolor image. Typically, this process consists of the steps of uniform electrostatic charging of a photoconductive insulating layer by means, for example, of corona discharge under subdued light. The photoconductive layer may be a photosensitive paper such as Electrofax paper, for example, which is composed of an electroconductive paper substrate and a photoconductive layer thereover of powdered zinc oxide in an insulating resinous binder. The photoconductive layer is subjected to imagewise exposure of a light and shadow pattern to form an electrostatic latent image thereon which image is developed by contacting the surface of the photoconductive layer with a liquid developer thereby rendering the latent image visible.

While capable of producing satisfactory reproductions, this liquid development process is not free from difiiculties. As mentioned above, the development speed is low in order to maintain adequate image density. In addition the presence of the edge effect renders it impossible to provide reproductions of originals having uniform density in wide portions.

The edge effect is caused by the distribution of the electrostatic field formed by the electrostatic charge pattern of the electrostatic latent image on the surface to be developed. Where the electrostatic charge is distributed uniformly over a wide area, the electric field becomes stronger at the peripheral portion and weaker at the central portion. Thus, the component of said :field perpendicular to said surface is not constant within this area, but exhibits a maximum and minimum respectively on the periphery and center of said area. When a sheet provided with such electrostatic image is brought into contact with liquid developer, the development proceeds rapidly in the peripheral portion and slowly in the center portion which produces an uneven density of toner particles. This effect is the so-called edge effect. Theoretically edge effects can be removed by prolonging the development time sufficiently. This, however, is not realizable in actual conditions because of dark-decay of the electrostatic latent image during the course of development. In addition, prolonging the developing time is not desirable for practical purposes, since to increase the time results in an overall undesirable slow reproduction rate.

The use of a development electrode placed in the proximity of the imaging surface during development has been proposed to remove the edge effect. Theoretically smaller distance between the imaging surface and the development electrode leads to improved proportionality between the perpendicular component of electrostatic field and the actual distribution of electrostatic charge and therefore, results in a smaller edge effect. In actual circumstances, however, this is not obtained because low developed image density is obtained due to the insufficient supply of liquid developer as a result of short development time or darkdecay of the electrostatic image during prolonged development. Furthermore, eventual contact between flexible imaging surface and the development electrode will result in the destruction of toner image formed during development. These factors have made it very difficult to remove the edge effect completely by reducing the distance between the imaging surface and the development electrode.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a liquid development system which overcomes the above noted deficiencies.

It is another object of this invention to provide a liquid development system providing improved solid area coverage.

It is another object of this invention to provide a liquid development system capable of increased development speed.

It is another object of this invention to provide a liquid development system which produces developed images with substantially no edge effect.

It is another object of this invention to provide a liquid development system which produces high density, nonstreaky developed images.

The above objects and others are accomplished, gen erally speaking, by providing an electrostatographic imaging system of the liquid development type wherein a liquid developer of elevated viscosity is initially supplied to the surface of an electroconductive developer applicator device followed by the application of a liquid capable of lowering the viscosity of the liquid developer. The electrostatographic imaging member is thereafter maintained in moving contact with the electroconductive applicator device for suflicient time to insure adequate development. Typically, the viscosity of the liquid developer may be lowered by the application of a liquid capable of dissolving a resin or varnish contained in the liquid developer.

The invention may be further illustrated by reference to the accompanying drawings in which:

FIG. 1 is a schematic sectional side view of the apparatus according to the present invention.

FIG. 2 is a view in perspective of an example of an electroconductive developer applicator drum adapted for use in the apparatus shown in FIG. 1.

In the development system shown in FIG. 1, an electrophotographic photosensitive continuous sheet 1 bearing an electrostatic latent image thereon is passed by guide roller 2 and a pair of pinch rollers 3 and 4. The lower portion of roller 4 is immersed in a prebath solution 6 contained in a reservoir and uniformly applies the prebath solution to the image bearing surface of said sheet 1. At least the periphery of roller 4 should be made of an insulating material capable of holding the prebath solution. Typically, this roller can be formed of a metal core covered with spongelike material or highly insulating resin. The prebath solution is employed to uniformly moisten the surface to be developed before contact with the liquid developer in order to prevent fogging. Any suitable prebath liquid may be employed. Typically, a highly insulating colorless liquid inert to the photosensitive layer, such as nonpolar hydrocarbon is employed. Typical prebath liquids include kerosene, cyclohexane and Isopar H, an isoparaffinic material available from Humble Oil and Refining Company. At least one of rollers 3 and 4 are preferably made of soft elastic material (rubber or sponge, for example) in order to provide uniform contact between the image bearing layer and the roller 4.

This development system further contains a liquid developer applicator device such as the relatively large diameter electroconductive rotary drum 10, of FIG. 1. A roller 11 applies liquid developer 15 of relatively high viscosity from a reservoir 14 to the periphery of the electroconductive rotary drum 10. A drum '17 supplies in sulating liquid 18 from a reservoir 19 to the liquid developer held on the periphery of the electroconductive rotary drum 10.

The continuous photosensitive sheet 1 bearing an electrostatic latent image is uniformly moistened with prebath solution 6 and is then brought by means of a guide roller 12 into close contact with the periphery of the drum 10, and then conveyed in moving contact at the same speed with the drum over a portion of the drum sufficient to provide the necessary time to achieve adequate development. While the developer applicator device may be of any suitable configuration such as a moving belt or web the use of the rotary drum is particularly preferred since it minimizes the machine space necessary to provide the prolonged moving contact between the de veloper applicator surface and the electrophotographic recording material. In addition, the desired uniform contact and pressure between the recording material and developer applicator surface is easier to maintain with an applicator device in the form of a rotary drum. To achieve maximum benefits from the use of a rotary drum applicator device, the recording material is conveyed in moving contact with the rotary drum of suitable diameter over a length at least half of the periphery of the drum by rotation of electroconductive rotary drum and recording material. The recording material is separated from the rotary drum and is passed over guide roller 13. The periphery of the drum can be mirror finish or it can be provided with minute irregularities such as matte finish. Furthermore, the drum 10 may be provided with steps as shown in FIG. 2 in order to form a thin film of the liquid developer between the image holding surface and the surface of the drum 10.

Liquid developer 15 of relatively high viscosity is applied by roller 11 to the periphery of the large diameter electroconductive rotary drum 10'. The concentration of toner in the liquid developer applied to and held on the periphery of the rotary drum is that which is necessary to have the toner provide the maximum image density and be almost completely attracted from the liquid developer to portions of the imaging surface of maximum electro static potential. Since this amount of toner particles must be available in the liquid developer held on the periphery of the surface the concentration is high and the liquid developer therefore is highly viscous. While the use of such a highly viscous liquid enables the periphery of the developer applicator device to hold sufficient amount of the liquid developer to provide maximum image density, it also decreases the migrating speed of toner particles to the imaging surface. As a result of the decreased speed of migration in such highly viscous liquid developer, a lengthened development period or intensified electrostatic field of the latent image or an extremely short distance between the image bearing surface and development electrode to minimize the migration of toner particles is generally required. However, extending the development period or intensifying the surface potential beyond certain limits are generally avoided. in addition, the extremely short distance between the image bearing surface and the development electrode inevitably leads to the lack of adequate supply of liquid developer which results in low image density.

According to the present invention, the viscosity of the liquid developer held on the surface of the developer applicator such as the circular drum is lowered at the surface of the liquid developer layer by applying an insulating liquid 18 of lower viscosity and which may be capable of dissolving the resin or varnish contained in the liquid developer. Typical materials present in the liquid developer which are capable of being dissolved include modified phenol formaldehyde resin, modified soybean oil and sunflower oil alkyd resins and linseed oil varnish. This insulating liquid may be applied in any suitable manner. Typically, it may be applied by a roller 17 rotating partially immersed in a bath 19 of the liquid 18. Consequently, the viscosity of developer is sufficiently reduced While the developer applicator device such as the drum 10 is contacting the photosensitive sheet 1. As a result, the migrating speed of toner at the outer surface of the developer layer is accelerated and development speed is increased. Development is carried out while the electrophotographic recording sheet 1 is kept in contact with the liquid developer, during which the periphery of the developer applicator device functions as the development electrode.

Any suitable insulating liquid may be employed to reduce the viscosity of the liquid developer. Typical materials include cyclohexane, methylcyclohexane, toluene, xylene, trichloroethylene or the mixtures thereof. Characteristic of this invention is the use of a highly viscous liquid developer which is easily held in sufiicient amount on the periphery of the developer applicator device and which is altered to a lower viscosity during development to thereby increase the mobility of toner particles. The selection of liquid developers and insulating liquids of appropriate viscosity may readily be made by the artisan for any specific machine configuration and performance characteristics. Typically, the viscosity of the liquid developer at about room temperature is in the range of from about 4 to about 10 centipoises while that of the insulating liquid is from about 2 to about 4 centipoises when operating at development speeds on the order of from about 100 millimeters per second to about 250 millimeters per second.

Following separation of the electrophotographic recording member and the developer applicator device, the toner not attracted to the recording member by the electrostatic field and remaining on the aplicator device may be removed from the applicator device by any suitable technique just as by being scraped off by a blade 16 and recovered.

In FIG. 1, the photosensitive sheet 1 developed during contact with the drum 10 is guided by the guide rollers 13, 21 and 22 and then passed through washing section where washing solution is sprayed from nozzles in pipes 20 to wash excessive developer from the surface of photosensitive sheet 1. The washing solution after washing is recovered by the receiver 26 and recycled to the pipes 20 by means of a pump not represented in the drawing. Any suitable liquid may be employed as the washing liquid. Typically, the washing liquid is low boiling, quick drying and has a low dissolving power. Typical materials include Isopar E, an isoparafiinic material available from Humble Oil and Refining Company and ethane hexachloride. After washing, the photosensitive sheet 1 is squeezed by the rollers 23 and 24 and then taken up on a reel. The squeeze roller coming into contact with the image bearing surface of the sheet as a hard, smooth surface, and can be composed, for example, of a mirror finish metal or glass roller.

The developer applicator such as the drum 10 can be provided with step or sleeves 102 as shown in FIG. 2, in which a portion 101 of the drum is the development electrode and portions 102 hold the edges of photosensitive sheet. The drum may be driven about its axial shaft 103 by any suitable means. The step between the portions 101 and 102 is provided in order to form a small spacing between the development electrode and the image bearing surface and to prevent the leakage of liquid developer from the edge portions of the photosensitive sheet 1. The difference in diameter between portions 101 and 102 should be less than about 500 microns. For maximum effectiveness of the development electrode, the spacing is preferably between about 30 and about 200 microns. When using a drum with a step structure, the width of roller 11 is preferred to correspond to the width of the recessed portion 101 of the drum.

The prebath solution may be dispensed with in some cases, where, for example, the insulating liquid 18 can also be used as prebath liquid. If provided with suitable properties which will not destroy the electrostatic latent image present on the imaging member, the insulating liquid 18 when supplied to the surface of the high viscosity liquid developer comes into contact with the photosensitive sheet to function as a prebath solution before it mixes with said liquid developer.

From the above description of the invention the choice of specific materials and operating conditions is deemed to be well within the scope of those skilled in the art and therefore the scope of the invention is not limited by the hereinabove mentioned illustrative materials. For example, while photosensitive paper comprising zinc oxide and an insulating binder layer are discussed above, it is to be understood that other imaging members may be employed and that the choice of particular imaging member and particular development system may be readily determined by one skilled in the art. For example, cadmium sulfide, zinc sulfide, zinc selenide, cadmium selenide, titanium dioxide, phthalocyanine and polyvinyl carbazole may be employed as a photoconductive material. In addition, other suitable electrostatographic imaging members may be employed. Development of the electrostatic latent image may be obtained with any suitable liquid developer. Typi-' cal liquid developers contain electroscopic marking particles dispersed in an insulating liquid vehicle and may also contain control agents and suspending agents for their well known functions. The liquid employed must have a relatively high insulating value, generally having a volume resistivity greater than about 10 ohm-cm. so as not to affect the electrostatic charge pattern on the insulating layer and low dielectric constants of less than about 3.4. Typical specific vehicles include hydrocarbons such as benzene, xylene, hexane, naptha, kerosene, halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene and chloroform. Typical electroscopic marking particles include among others charcoal, carbon black, magnesium oxide, lithopone, cadmium yellow, chrome yellow, cobalt blue, cadmium red, burnt sienna, Hansa yellow, rose bengal and phthalocyanine. Typically, the electroscopic marking particles are present in an amount of from about 4 to about 20 grams per liter. The electroscopic marking particles are conventionally dispersed and suspended in the liquid by stirring or agitation and where a highly uniform and stable suspension is deslilriil, the suspension may be passed through a colloid The present invention provides a method and apparatus for developing electrostatic latent images to provide reproductions of high image density by applying a sufiicient quantity of highly viscous liquid developer to the periphery of a developer applicator device followed by decreasing the viscosity of the liquid developer to increase the moblllty of toner particles before the liquid developer comes into contact with the recording member bearing an electrostatic charge pattern to be developed. At the very least, the addltion of a layer of a less viscous liquid to the developer applicator device adjacent the recording member provides a medium capable of enabling rapid transport of marking particles therethrough. In addition, the edge effect is minimized by utilizing the developer applicator device such as a rotary drum as the development electrode. Furthermore, streaking of the developed image is eliminated by maintaining the periphery of drum and liquid developer and the image bearing surface fixed with respect to each other during development. In a particularly preferred embodiment, the photosensitive sheet is kept in contact with the drum over at least a half of the periphery of the drum in order to use as small a drum and as high a rotation speed as possible thereby increasing the processing speed while maintaining sufficient developing period.

Although particular embodiments have been set forth using the development system and technique of this invention, these are merely intended as illustrations of the present invention. There are other systems and techniques which may be substituted for those described. Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure which modifications are intended to be included within the scope of this invention.

What is claimed is:

1. A method of developing an electrostatic latent image present on an electrostatographic imaging surface comprising providing a liquid developer applicator surface with a substantially uniform layer of an insulating liquid developer of a first higher viscosity having a toner particle concentration sufficient to provide maximum image density in portions of maximum electrostatic potential, applying a layer of an insulating liquid to said first layer of liquid developer, said insulating liquid being of a viscosity sutficiently lower than said liquid developer to lower the viscosity of the liquid developer and increase the mobility of the toner particles in the liquid developer, contacting said imaging surface bearing said electrostatic latent image with said liquid developer present on said developer applicator surface and maintaining said imaging surface and said liquid developer in moving contact so that there is substantially no difference in speed between said imaging surface and said liquid developer for a time sufficient to develop said electrostatic latent image, and separating said imaging surface and said developer applicator surface.

2. The method of claim 1 including the step of prebathing said electrostatographic imaging surface with an insulating liquid prior to contacing said imaging surface with said liquid developer.

3. The method of claim 1 wherein said electrostato graphic imaging surface comprises a photoconductive insulating layer.

4. The method of claim 3 wherein said photoconductive insulating layer comprises zinc oxide particles in an insulating resin matrix.

5. The method of claim 1 wherein said developer applicator surface also serves as a development electrode and is maintained out of contact with said electrostatographic imaging surface.

6. The method of claim 5 wherein the spacing between the electrostatographic imaging surface and the developer applicator surface is less than about 500 microns.

7. The method of claim 1 wherein said developer applicator surface comprises a surface of a rotary cylindrical drum.

8. Apparatus for developing an electrostatic latent image present on an electrostatographic imaging member comprising an electroconductive developer applicator device, means to apply a substantially uniform layer of a highly viscous liquid developer to the surface of said applicator, means to apply an insulating liquid to said layer of liquid developer to lower the viscosity of said liquid developer, means to engage said imaging member and said electroconductive applicator device such that they are not in contact but the surface of said imaging member is in moving contact with the liquid developer present on the surface of the applicator, means to maintain said moving contact for a period of time sufiicient to develop said electrostatic latent image, and means to separate said imaging member and said developer applicator.

9. Apparatus according to claim 8 wherein said developer applicator device comprises a rotary mounted circular drum.

10. Apparatus according to claim 8 wherein said developer applicator device is provided with a positioning member at the ends thereof to maintain the spacing between the developer applicator surface and the imaging member.

References Cited UNITED STATES PATENTS 3,247,007 4/1966 Oliphant l17-37 3,512,965 5/1970 Matkan 117-37 X 3,003,404 10/1961 Metcalfe et a1 35510 3,405,683 10/1968 Ions et al 11737 X 3,093,052 6/1963 Burner et al. -94 2,707,916 5/1955 Smith et al. l01-363 3,368,526 2/1968 Matsumoto et al. 118637 WILLIAM D. MARTIN, Primary Examiner E. J. CABIC, Assistant Examiner U.S. Cl. X.R.

ll793.4 A, 111 R; 118246, 258, 262, 637, Dig. 23 

